Conditioned atmosphere furnace muffle



Dec. 10, 1968 J. H. BECK CONDITIONED ATMOSPHERE FURNACE MUFFLE Filed Aug. 18, 1967 INVENTOR. J. HOWARD BECK W ATTORNEY Y B J mm @z m mm v m U N Ob Q II I C K 1 v til. 11 fgflhm NMN i J 6528 EEG v 5 mm m mmwbmr mzoN mm P llllll ll. mm \mm mm L 6 mm mm 621$ fizziu E United States Patent 3,415,503 CONDITIONED ATMOSPHERE FURNACE MUFFLE Jacob Howard Beck, Wuhan, Mass., assignor to BTU Engineering Corporation, Waltham, Mass, a corporation of Massachusetts Filed Aug. 18, 1967, Ser. No. 661,652 11 Claims. (Cl. 263--8) ABSTRACT OF THE DISCLOSURE Field of the invention The present invention relates in general to furnaces having particular utility in the processing of thick-film microelectronic or integrated circuit elements and related devices where exceedingly close control of temperature and atmosphere is essential to successful performance. It is a feature of the present invention that nonturbulent or laminar gas flow may be established throughout a relatively lengthy muffle whereby products transported therethrough will not be subject to significant temperature variations axially or transversely of the direction of motion. As will be seen, nonturbulent gas flow is achieved over a substantial length of the furnace mufile by means of the introduction of high pressure gas in a venturi-like section of the structure and that this high pressure \gas in turn may be used to create a limited region of turbulent flow which may be used to advantage in the processes under consideration.

Discussion of the prior art The mass production of integrated circuit elements has been limited by many problems resulting from uneven heat treatment in the furnace used to reduce the various wet inks to conductive and resistive lines and areas on a ceramic substrate. One difficulty which has hampered volume product-ion has been the inability to maintain temperature uniformity in a region of gas flow chiefly due to the fact that eddies in the gas stream resulted in significant temperature variations, both in the direction of transport through the furnace and transversely across the conveyor belt. Significant temperature differences in any direction result in product variances which are intolerable with the result that yields have been relatively low, while the cost of the product has been high. While many efforts have been made to achieve temperature uniformity in an atmosphere of flowing gas, discontimities, pressure variations and the like have always resulted in substantial quality variations from piece to piece. In addition to the foregoing, problems have risen in prior furnaces in the disposal of the injurious, if not poisonous fumes generated during the drying and firing processes. Exhaust fans ordinarily used to vent fumes through an external stack normally add turbulence in the gas stream and tend to increase the variation of temperature in the system. In summary, prior art furnaces used for such processes have represented design by compromise to achieve reasonable utility.

Summary of the invention.

The present invention concerns a furnace for heat treatment of specific products, such as integrated circuits, having an elongate tubular muflle with a product input end flared outwardly in the form of a venturi. Relatively high pressure gas is introduced at the venturi throat, the action of which is to draw low pressure gas from the opposite end of the muflle over the work substantially without turbulence. A second venturi and gas input arrangement at the product input end of the muflle exhausts gas drawn through the length of the mufile as aforesaid, gas introduced at the throat of the first-mentioned venturi and such vapors as may be emitted from the work inprocess. Axial sections of the muflle may be maintained at different temperatures and turbulent gas flow in the venturi section may be utilized to enhance the treatment of products in that limited area.

Description of the drawing FIG. 1 is an axial cross sectional view of a furnace embodying the novel features of this invention; some of the more conventional structural aspects being shown in fragmentary and diagrammatic form;

FIG. 2 is a cross sectional view of the novel furnace taken along the plane 2-2 of FIG. 1 illustrating the gas input arrangement at the exit-end of the furnace muffle; and

FIG. 3 is a cross sectional view of the novel furnace taken along the plane 3-3 of FIG. 1 illustrating the gas jets in the venturi section of the muflle.

D scription of the preferred embodiment With reference now to the drawing and more particularly to FIG. 1 thereof, there is shown in cross sectional view a furnace particularly adapted to the firing of relatively small distinct elements, such as integrated circuits in a precisely conditioned atmosphere. Generally speaking,'the furnace of the present invention is comprised of an elongate muflle 10 having an open input-end 11 and an open exit-end designated by the reference numeral 12. The cross sectional shape of the muflle 10 in the central region is clearly shown in FIGS. 2 and 3, and as is ap parent, the muflle is formed 'with three flat sides and a peaked upper portion. In the input region 11, the muflle is of substantially rectangular cross-section, flaring outwardly in the vertical dimension to define a venturi with a throat or constriction 11a for purposes to be set forth below. The muflle is preferably made of a material which is chemically resistant to the fumes and vapors present in high temperature fabrication of products such as integrated circuits. A suitable material is quartz; al though high temperature stainless steels such as Inconel may be also employed in some applications.

Although the various detailed features of the present invention are closely related to and embodied in the structure of the mufile 10, certain aspects of a complete furnace embodying this muflle should be described. In partioular, muffie 10 is enclosed within a suitably insulated structure built up, for example, of the brick 14. The muffle 10 may remain in a horizontal position, or if desired it may be tilted slightly, with the input-end 11 raised a few degrees to enhance axial gas flow. Since the means for bringing the muffle zones to suitably precise operating temperatures, such as heating coils, temperature sensors and control means therefor, are conventional and readily available in the art, these have not been shown in the drawing, other than diagrammatically, for the sake of clarity.

As shown in FIGS. 1, 2 and 3, a conveyor belt 21, which may be made of flexible stainless steel mesh or hinged links, passes lengthwise through muflle 10 close to the lower flat wall. The conveyor is driven by a pair of rotatable pulleys 22 and 23, turned by a source of power (not shown) and transports the work pieces in process into, through and out of the furnace in conventional fashion; the process speed being dependent upon the parameters of the items being fired, process temperatures, and the axial length of the furnace. For the purpose of illustrating typical furnace operation the conveyor belt is shown in each of the figures as supporting a plurality of integrated circuit elements 25, each being essentially a ceramic substrate having a circuit pattern of conductors, resistors and the like imprinted thereon with appropriate inks as established in that art. This will be discussed in somewhat further detail below.

The manner in which the individual microelectronic circuit elements are deposited upon conveyor belt 21 at the input-end and removed from the conveyor belt 21 at the output-end does not constitute an element of the present invention. For completeness, however, it should be mentioned that various automatic machinery is available for placing the elements on the conveyor and removing the completed products at the output for further processing elsewhere. For an understanding of the present invention it will be sufiicient to recognize that with the conveyor belt in operation, motion being in the direction of the arrows, the integrated circuit elements are conveyed axially through the furnace from the input-end to the output-end at a uniform speed and are thus exposed to the various process temperature zones and other conditions which exist in the mufiie in the path of motion. More specifically, the various work pieces being processed in passage through mufile 10 are exposed not only to various elevated temperatures but also to an atmosphere of gas which flows continuously from the exit-end to the inputend. For the purpose of confining the flow of the process gases to the region within the mufile, both the input and exit ends have been provided with gas barriers 31 and 32 which essentially consist of a series of transverse parallel metal plates extending from the upper inner surface of the mufiie to a point just out of contact with the work pieces being transported on conveyor belt 21.

For the furnace shown in the drawing, two distinct process temperature zones have been provided along the axial dimension of the mufile. As shown, the input section 11 is operated at a temperature T while the longer section to the right is operated at temperature T The exact temperature levels are of course dependent upon the process requirements. In typical use for integrated circuit processing, the input section 11 would be maintained at a temperature T suited to burning out the liquid vehicles and solvents in the various inks used. Temperature T would be higher and would serve to fire the metallic constituents of the inks to the desired finish.

In order to maintain the two sharply defined temperature regions, a transverse heat barrier designated generally by the reference numeral 35 is inserted between mufile sections. Structurally the heat barrier consists of three relatively large diameter circular parallel plates of material of high thermal conductivity joined at their peripheries by a circular pipe 41. The central plate 36 is formed with rectangular central opening 37 through which the conveyor belt and work pieces may pass. Outer plates 38 and 39 are rigidly secured to and are formed with openings equal in size to their respective mufile sections. Pipe 41 is provided with inlet and outlet ports 42 and 43 respectively through which a coolant such as water under pressure may be circulated. Heat flowing from the adjacent sections of the muifie through plates 38 and 39 will be dissipated by the coolant, while central plate 36 will inhibit thermal energy interchange between muflie sections by radiation. In operation with a coolant flowing, the heat barrier 35 will maintain a sharp temperature gradient across its small axial dimension, while conventional operation may be restored simply by discontinuing the flow of cooling medium. A similar heat barrier is the subject of United States Patent No. 3,041,056 dated June 26, 1962, and reference is made to that patent for further details of design and operation.

With the foregoing structural features of the furnace and the furnace mufile in view, the arrangement for controlling the gas atmosphere will now be described in detail. Turning first to FIGS. 1 and 2, the mechanism for introducing a specific gas into the exit-end of the muffle is illustrated. More specifically the arrangement consists of a gas inlet tube 51 which extends inwardly through fire brick 14 and through a suitable opening in the upper portion of muffie 10 where it is terminated by a horizontal connecting tube 52 having closed ends and formed with a series of gas ports 53 directed toward the input end 11. As is thus evident, gas entering the upper portion of tube 51 will flow outwardly toward the forward end of the muffle through ports 53.

At the input-end of the mufile a generally similar gas flow structure is provided in the form of a gas input tube 55 which extends radially inward through gas barrier 35 and is terminated as indicated best in FIG. 3 in a transverse tube 56 having closed ends and a plurality of gas ports 57 which face forwardly into muffie input section 11. The center plate 36 of the gas barrier lies immediately to the right of transverse tube 56, at the venturi throat 11a.

As shown in FIG. 1, input section 11 of mufile 10 is provided at its forward end, just inwardly of gas bafiles 31 with a vertical gas exhaust section 61 made up of a forward flat wall 62 and a shaped wall 63 arranged to define a second venturi throat 64. A gas line 65 extends downwardly into mufile 66 and curves upwardly at 67. terminating in a jet 68 pointed into venturi throat 64.

In the processing of integrated circuit elements, critically controlled temperatures and atmospheres are absolutely essential to successful results. In the production of integrated circuit elements the substrate may consist of a high density ceramic, such as aluminum oxide, por celain, beryllia, or the like. This base material is coated, usually by automatic screening machinery, with thickfilm resistor and/or conductor compositions, which are ordinarily mixtures of powdered metal and/or glass particles suspended within a liquid organic vehicle. By whatever process used in their preparation the integrated circuit elements 25 at the time of deposit upon the input end of conveyor 21 have a number of wet ink lines and areas on the upper surface thereof. These inks are to be burned-out and then fired.

In the input section 11 of the muffie shown in FIG. 1 the temperature T to which the elements 25 are subjected is suificient to burn-out the inks prior to their passage through the heat barrier to the higher firing temperature T zone. It is with this process technology in view that the gas atmospheres and gas flow in the furnace shown in FIG. 1 will now be discussed.

Considering first the process of firing at temperature T it is absolutely essential that local temperature variations be avoided both axially and transversely of the conveyor belt. Gas eddies and turbulent gas flow in any form has the property of creating substantial temperature variation, while laminar, non-turbulent flow results in the virtual elimination of temperature gradients in a well constructed furnace.

In the furnace of the present invention laminar gas flow is achieved in the mufile between gas ports 53 and the opening 37 in the central plate 36 of the heat barrier by connecting tube 51 to a source of gas at relatively low pressure. Indeed, the gas may be air at atmospheric pressure, preheated to temperature T and dried to avoid adverse effects in the furnace. The gas entering the muffle through ports 53 is drawn toward section 11 by the action of gas introduced through ports 57 at the venturi constriction, at relatively high pressure through tube 55. The fact that gas is introduced through tube 51 with little or no positive pressure while gas at high pressure is introduced forwardly through ports 57 results in smooth, laminar gas flow throughout the entire length of mufile 10 between ports 53 and the opening 37. This atmosphere is ideally suited to the processing of micro-electronic elements 25 at firing temperature T At this point it should be observed that the high pressure gas introduced through ports 57 toward the forward end of the muffle is relatively turbulent in region 11 (as indicated by the curved arrows on the drawing). This agitation of gas is quite useful in burning-out the inks at temperature T and also serves to prevent carburization at the surface of the integrated circuit substrates 25.

Turning now to the vertical venturi defined by throat 64 between plates 62 and 63, it will be observed that high pressure gas entering through tube 65 and directly upwardly through nozzle 68 has the efiect of exhausting gas and vapor contained in mufi le section 11. As indicated in FIG. 1, the exhaust region 61 is preferably connected to a chimney or stack which is vented away from all personnel since the hydrocarbon vapors of the drying vehicles in muffle section 11 may be injurious or even poisonous. The combined action of the vertical venturi and gas bafiles 31 precludes gas exhaust through the open input-end of the mufile.

In the processing of thick-film integrated circuits, dry air may be used as the gas introduced through all three tubes 51, 55 and 65. As previously described, the gas introduced through tube 51 may be preheated to temperature T to eliminate local temperature profiles while the gases introduced through tubes 55 and 65 may be somewhat cooler. It is of interest to note that through the selective use of varied gas pressures, both laminar and turbulent fiow may be achieved in closely adjacent sections of the muflie, each type of flow serving a highly useful purpose.

Gas entering through tube 51 serves to maintain temperature uniformity and provides the proper chemical environment during the firing process of temperature T gas entering through tube 55 serves to draw the gas forward from ports 53 and also to provide turbulent burnout in the input section; and gas entering through tube 65 serves to exhaust all fumes from the mufile and to reduce back pressure in the region 11 which might otherwise be present as a result of high pressure gas entry through ports 57. Gas flow in the specified direction thus prevents flow of vapors of the volatile binders and decomposition products into the region of temperature T where contamination of the product might result.

While the invention has herein been described for the processing of thick film integrated circuit elements in two temperature zones under dry air, it should be mentioned that this furnace may be used for heat treatment of various other products. The gases which are introduced may be varied and may be chemically active or inert, or dry or moist, the temperatures in the respective zones may be changed or may be made equal, and the speed and other controls may be altered to achieve a desired purpose. Thus, this invention is to be construed only in accordance with the scope of the appended claims.

What is claimed is:

1. A furnace adapted for the continuous heat treat ment of specific products comprising:

an elongate tubular mufile having product input and exit ends;

a conveyor extending axially through said mufiie for transporting said products therethrough;

and gas input and exhaust means disposed respectively at said exit and input ends for providing substantially means for providing gas at relatively high pressure to said venturi section of said mufile in said exhaust means.

3. A furnace in accordance with claim 2 wherein said gas provided to said gas input means is at a first temperature while said gas provided at said exhaust means is at a second and diiferent temperature.

4. A furnace in accordance with claim 3 wherein the temperature maintained in said muffle in the region of said venturi section is different than the temperature maintained throughout a substantial axial section of said muflie in the region between said venturi and said gas input means.

5. A furnace in accordance with claim 4 wherein said gas provided at said exhaust means is arranged to cause turbulence in said venturi section.

6. A furnace adapted for the continuous heat treatment of specific products comprising:

an elongate tubular muffle having product input and exit ends, a section of said mufile adjacent said input end being formed as a venturi, said venturi having a throat at its juncture with the remainder of said rnufiie and being flared outwardly toward said input end;

a conveyor extending axially through said muffle for transporting said products therethrough;

means adjacent said exit end for introducing gas into said muffie at relatively low pressure with flow directed toward said input end; and

means arranged in the throat of said venturi for introducing gas at relatively high pressure with flow directed toward said input end thereby to draw gas introduced in the region of said exit end axially through said muffie substantially without turbulence.

7. A furnace in accordance with claim 6 and including gas barriers within said input and output ends of said muflle to limit the flow of gas through said ends substantially without interference with said conveyor.

8. A furnace as in claim 6 and including:

a heat barrier adapted to define distinct temperature zones in said venturi section and the remainder of said mufile;

said means for introducing gas into said throat of said venturi being formed with a gas conveying tube extending into said mufile through said heat barrier.

9. A furnace in accordance with claim 6 and including:

a gas exhaust system operatively coupled into said venturi section of said muffle and arranged to exhaust said gas introduced into said venturi section and said gas introduced at said exit end of said muffie.

10. A furnace in accordance with claim 9 wherein:

said gas exhaust system includes a second venturi section extending outwardly from said venturi section of said mufile;

and means for introducing gas into said second venturi section in the direction in which gas is exhausted from said furnace.

11. A furnace in accordance with claim 10 wherein said gas introduced in said second venturi is directed substantially at right angles to the direction of gas introduced into said venturi section of said muflle.

References Cited UNITED STATES PATENTS JAMES W. WESTHAVER, Primary Examiner.

US. Cl. X.R. 26315, 37, 50 

